The present invention relates to a method to prepare cyclopropenes.
Cyclopropenes have a variety of uses in the chemical industry. One recent discovery is the ability of 1-methylcyclopropene, and related analogs, to inhibit the action of ethylene on plants (see U.S. Pat. No. 5,518,988). Several syntheses for cyclopropene and its simple derivatives have been reported. The most widely practiced process is the sodium amide induced xcex1-elimination of an allylic chloride (see F. Fisher and D. Applequist, J. Org. Chem., 30, 2089 (1965)). Unfortunately, this method suffers from relatively low yields, difficulty in purifying the final product, and the use of excess sodium amide. An improved synthesis of 1-methylcyclopropene incorporates the use of freshly prepared halide free phenyllithium as the base replacing sodium amide. This method provides overall yields in the 60 to 80 percent range (see R. Magid, et. al., J. Org. Chem., 36, 1320 (1971)). Similar synthesis methods are disclosed in U.S. Pat. No. 6,017,849.
Although the reported reactions work reasonably well, a major problem often encountered is the presence of unwanted isomers such as methylenecyclopropane in the final product mixture. Methods which improve the isomer ratio (that is, increased 1-methylcyclopropene compared to the methylenecyclopropane often do so at the expense of overall yield of product (see Koster, et. al., Liebigs Ann. Chem., 1219 (1973)). Because of these problems, there is still a need for a method to prepare 1-methylcyclopropene in high yield and in high isomer purity. I have discovered that when a catalytic amount of certain weaker bases is added to the reaction mixture of a substituted or unsubstituted allyl halide and a strong base in an inert solvent, cyclopropenes, with greatly reduced unwanted isomer content, are produced in high yield.
The present invention, therefore, is a method to prepare cyclopropenes, comprising combining an allyl compound of formula I: 
wherein:
X is a leaving group; and
R is hydrogen or a substituted or unsubstituted alkyl, alkenyl, alkynyl, cycloalkyl, cycloalkylalkyl, phenyl, or naphthyl group; wherein the substituents are independently halogen, alkoxy, or substituted or unsubstituted phenoxy;
with a non-nucleophilic strong base in an inert solvent in the presence of a catalytic amount of a weaker, non-nucleophilic base.
As used herein, the term xe2x80x9calkylxe2x80x9d means both straight and branched chain (C1-C20) radicals which include, for example, methyl, ethyl, n-propyl, isopropyl, 1-ethylpropyl, n-butyl, tert-butyl, isobutyl, 2,2-dimethylpropyl, pentyl, octyl, and decyl. The terms xe2x80x9calkenylxe2x80x9d and xe2x80x9calkynylxe2x80x9d mean (C3-C20) alkenyl and alkynyl groups such as, for example, 2-propenyl, 2-butenyl, 3-butenyl, 2-methyl-2-propenyl, and 2-propynyl. The term xe2x80x9ccycloalkylalkylxe2x80x9d means a (C1-C15) alkyl group substituted with a (C3-C6) cycloalkyl group such as, for example cyclopropylmethyl, cyclopropylethyl, cyclobutylmethyl, and cyclopentylethyl. The term xe2x80x9chaloalkylxe2x80x9d means an alkyl radical wherein one or more of the hydrogen atoms have been replaced by a halogen atom. The term xe2x80x9chalogenxe2x80x9d means fluorine, chlorine, bromine, and iodine. The term xe2x80x9cstrong basexe2x80x9d means a non-nucleophilic base with a pKa greater than 35. The term xe2x80x9cweaker basexe2x80x9d means a non-nucleophilic base with a pKa of from 26 to 35, or the conjugate acid thereof. The term xe2x80x9cinert solventxe2x80x9d means a solvent which does not react with the strong base, the weaker base, the allyl halide, or the resulting cyclopropene.
As used herein, all percentages are percent by weight, unless otherwise specified and are inclusive and combinable. All ratios are by weight and all ratio ranges are inclusive and combinable. All molar ranges are inclusive and combinable.
Preferably, X is a leaving group selected from halogen, alkyl or aryl sulfonyloxy, alkyl or aryl sulfate, and alkoxy. More preferably, X is chloro, bromo, iodo, benzenesulfonyloxy, p-toluenesulfonyloxy, methanesulfonyloxy, or t-butoxy. Even more preferably, X is chloro, bromo, or benzenesulfonyloxy. Most preferably, X is chloro or bromo.
Preferably, R is (C1-C10) alkyl. More preferably, R is (C1-C8) alkyl. Most preferably, R is methyl.
Preferably, the inert solvent is an aliphatic or aromatic hydrocarbon such as, for example, mineral oil, benzene, toluene, or xylene; an ether such as, for example, diethyl ether, tetrahydrofuran, or dioxane, a halogenated hydrocarbon such as, for example perhaloalkanes or methylene chloride, liquid ammonia, or lower alkyl amine or lower dialkyl amine such as, for example, methylamine or dimethylamine. More preferably, the solvent is an aliphatic or aromatic hydrocarbon. Most preferably, the solvent is mineral oil. The solvent may be a mixture of more than one inert solvent.
Preferably, the strong base is an alkali metal salt of an amine or an organometallic base. More preferably, the strong base is sodium, potassium, or lithium amide or phenylithium. Most preferably, the strong base is sodium amide. The strong base may be a mixture of more than one strong base. The amount of strong base used in the method will vary depending upon the weaker base used, the inert solvent, and the temperature at which the reaction is conducted. Preferably, the amount of strong base used is from 0.1 to 20 moles per mole of allyl compound of formula I. More preferably the amount of strong base used is from 0.5 to 2 moles per mole of allyl compound of formula I. Most preferably, the amount of strong base used is from 0.7 to 1.4 moles per mole of allyl compound of formula I.
Preferably, the weaker base is soluble in the inert solvent. More preferably, the weaker base is a silyl amine, a disilazane, their cyclic analogs, mixed cyclic silazane/ether analogs, or metal salts thereof. Even more preferably, the weaker base is a silyl amine or a disilazane. Still more preferably, the weaker base is a dialkyl- or trialkyl, diaryl- or triaryl, or mixed alkyl/aryl silyl amine; a tetraalkyl-, pentaalkyl- or hexaalkyl, tetraaryl-, pentaaryl-, or hexaaryl, or mixed alkyl/aryl disilazane; or their cyclic analogs. Still more preferably, the weaker base is 1,1,1-triphenylsilylamine, tri-n-hexylsilylamine, 1,1,1,3,3,3-hexamethyldisilazane, 1,1,3,3-tetramethyldisilazane, 2,2,4,4,6,6-hexamethylcyclotrisilazane, octamethylcyclotetrasilazane, hexaethyldisilazane, 1,3-di-n-octyltetramethyldisilazane, or 2,2,5,5-tetramethyl-2,5-disila-1-azacyclopentane. Most preferably, the weaker base is hexamethyldisilazane. The weaker base can be a mixture of more than one weaker base. The amount of weaker based used in the method of this invention will vary depending upon the strong based used, the inert solvent used, and the temperature at which the reaction is conducted. Preferably, the amount of weaker base used is from 0.001 to 0.95 moles per mole of strong base used. More preferably, the amount of weaker base used is from 0.02 to 0.4 moles per mole of strong base. Most preferably, the amount of weaker base used is from 0.02 to 0.2 moles per mole of strong base.
The order of addition of the reactants to the solvent is not critical. However, it is preferred that the allyl compound of formula I is added last to a mixture of the other components. Most preferably, the allyl compound is added slowly to a mixture of the other components.
The temperature at which the method of this invention is carried out is not critical. However, because cyclopropenes are reactive compounds, care must be taken to ensure either a) that the temperature is kept below that at which decomposition or side reactions occur, or b) the cyclopropene produced must be removed from the reaction mixture as it is being produced, or a combination of a) and b). Preferably, the cyclopropene produced will be distilled from the reaction mixture and collected in a cooled receiver as it is produced. For lower boiling cyclopropenes, the temperature is preferably greater than or equal to the boiling point of the cyclopropene and the cyclopropene is distilled from the reaction mixture as it is being produced. For higher boiling cyclopropenes, the temperature is preferably less than the decomposition temperature of the cyclopropene.
Preferably, the reaction mixture is stirred or otherwise agitated and/or sparged or purged with an inert gas during the reaction. Preferably the inert gas is nitrogen. More preferably, and particularly in the case of lower boiling cyclopropenes, the agitation rate is sufficiently high to ensure that the cyclopropene distills from the reaction mixture as soon as possible after being formed. Fast removal of the cyclopropene from the reaction mixture reduces production of side products such as alkylidenecyclopropanes and teleomers.
The pressure at which the reaction is conducted is also not critical. Ambient pressure is preferred. However, pressure or vacuum may be utilized to affect the relative boiling points of the cyclopropene produced and the inert solvent to aid in separation of the cyclopropene from the reaction mixture.